1. Field of the Invention
The present invention relates to a charge roller of a developer for use of an image forming apparatus adopting an electrophotographic printing system, and more particularly, to a charge roller of a developer for an image forming apparatus in which a semiconductive resilient rubber layer is formed of a double layer constituted by a foam rubber layer and a non-foam rubber layer, or a single layer of a cross linking rubber layer, to thereby minimize volume resistivity of the semiconductive resilient rubber layer, maximize crosslinking ratio of molecule of rubber, and suppress increase of durability of the semiconductive resilient rubber layer. Further, the present invention relates to a method of manufacturing such a charge roller and a receptor device employed for such a method.
2. Background Art
In general, an image forming apparatus adopting an electrophotographic printing system, for example, laser printer, facsimile apparatus, copier or similar image forming apparatus, is provided with a developer having a photosensitive drum and a developing roller. A photosensitive drum is formed with at an outer periphery thereof an electrostatic latent image via an exposer, and supplied with a toner via an adjacent developing roller, and develops the electrostatic latent image into a visual image using a toner, thereby forming an image onto a recording medium.
A conventional developer for an image forming apparatus is disclosed in U.S. Pat. No. 5,132,734 “Developing apparatus”, U.S. Pat. No. 5,260,748 “Electrostatic image developer dispenser”, U.S. Pat. No. “Developing device for an image forming apparatus”, U.S. Pat. No. 5,771,426 “Developing device using a toner and carrier mixture”, and U.S. Pat. No. 5,787,328 “Rotary developing device for an image forming apparatus”.
In such a conventional developer for an image forming apparatus, a charge roller is generally disposed in the vicinity of a photosensitive drum so as to rotate at a constant speed, being engaged with the photosensitive drum. Here, if the charge roller is applied with a voltage having a predetermined size, so-called “Paschen discharge” occurs, and the surface of the photosensitive drum which is in contact with the charge roller is electrostatically charged according to a relation among resistance and surface state of the charge roller and the voltage applied.
A conventional charge roller is disclosed in U.S. Pat. No. 5,499,078 “Charge roller and image forming apparatus using the same”, U.S. Pat. No. 5,600,414 “Charging roller with blended ceramic layer”, U.S. Pat. No. 5,768,653 “Electrophotographic printing device with a charging roller”, U.S. Pat. No. 5,792,533 “Electrostatic charging roller”, and U.S. Pat. No. 5,852,758 “Charge roller displacement mechanism”.
A conventional charge roller has a configuration in that a metallic rod has an outer periphery pressedly coated with a semiconductive resilient rubber with a low durability so that a photosensitive drum may have a uniformly charged surface.
Recently, it has been established through a variety of studies on the structure of a charge roller that a semiconductive resilient rubber should maintain a volume resistivity of 107 Ωcm to 108 Ωcm to allow a photosensitive drum to have a uniformly charged surface contacting a charge roller.
However, it is extremely difficult to produce a charge roller with an excellent quality unless an additional treatment is conducted to a semiconductive resilient rubber since a semiconductive resilient rubber constituting an outer surface of a charge roller has its own volume resistivity of 108 Ωcm or higher.
If a charge roller with a volume resistivity of 108 Ωcm or higher contacts a photosensitive drum, an excessively high voltage is required for charging the surface of the photosensitive drum. Thus, an amount of ozone generated by the applied high voltage significantly increases, resulting in a serious environmental pollution.
Considering such characteristics of semiconductive resilient rubber, a variety of techniques for reducing a volume resistivity of a semiconductive resilient rubber are sought in a conventional system.
For example, U.S. Pat. No. 5,637,395 “Powder coated charge roller” discloses a method of adding an additive like a conductive carbon powder or alkali metal salt to a semiconductive resilient rubber so as to reduce a volume resistivity of the semiconductive resilient rubber.
Another example shows a method of replacing a semiconductive resilient rubber by an acrylonitrile butadiene rubber or epichlorohydrin rubber.
In addition to the above-mentioned approaches for reducing volume resistivity of a semiconductive resilient rubber, some different approaches have been proposed for enhancing characteristics of a semiconductive resilient rubber.
For example, U.S. Pat. No. 5,497,219 “Charge rollers having improved layer structure and/or surface characteristics in an image forming apparatus” and U.S. Pat. No. 5,786,091 “Charge roller for an image forming apparatus” disclose a method of coating a surface of a semiconductive resilient rubber with polyamide, fluoric resin, or epichlorohydrin rubber liquid utilizing a spraying or dipping system so that the semiconductive resilient rubber may have a desirable thickness and surface roughness.
U.S. Pat. No. 5,248,560 “Filled urethane developer roller” describes a method of replacing a semiconductive resilient rubber by a polyurethane rubber having superior abrasion resistance and electrical characteristic.
However, many problems arise from those known approaches when applied to practice, as follows.
First, with the above-mentioned method disclosed in U.S. Pat. No. 5,637,395, it is extremely difficult to uniformly disperse an additive like a conductive carbon powder or alkali metal salt onto a semiconductive resilient rubber.
In this case, since the uniformity of the additive applied to the semiconductive resilient rubber is extremely poor, an overall volume resistivity of the semiconductive resilient rubber becomes non-uniform. Accordingly, an overall surface of a photosensitive drum contacting the semiconductive resilient rubber is non-uniformly charged, resulting in an undesirable image being produced.
In case where an additive is added, the durability of a semiconductive resilient rubber rapidly increases, and it will be difficult to maintain the durability of the semiconductive resilient rubber at 40 or less as prescribed by JISA (Japanese Industrial Standards type AK6301). Moreover, if a diameter of a charge roller is reduced at such a state, it will be extremely difficult to ensure a uniform contact between the charge roller and the photosensitive drum. As a result, it will be difficult to reduce the size of the charge roller.
The approaches presented in U.S. Pat. Nos. 5,497,219 and 5,786,091 require a coating process utilizing coating liquid like polyamide, fluoric resin, or epichlorohydrin rubber in addition to a process of forming a semiconductive resilient rubber at an outer periphery of a metal rod, thus significantly deteriorating an overall product process efficiency.
Moreover, the process of coating such liquid makes it difficult to maintain at a low durability of the semiconductive resilient rubber similarly to the case of adding an additive. This will also make it difficult to reduce a size of a charge roller.
The approach disclosed in U.S. Pat. No. 5,248,560 allows a relatively higher quality charge roller to be produced, it is still problematic from the overall production cost aspect since a high price polyurethane is employed for this approach.
If a semiconductive resilient rubber is replaced by an acrylonitrile butadiene rubber or epichlorohydrin rubber having a low volume resistivity, the completed product may have an outer rubber surface having a volume resistivity suppressed down to a level lower than a predetermined level. However, it will be extremely difficult to maintain the durability of the outer rubber surface at 10 or less as prescribed by JISA. Moreover, if a diameter of a charge roller is reduced at such a state, it will be extremely difficult to ensure a uniform contact between the charge roller and the photosensitive drum. As a result, it will be difficult to reduce the size of the charge roller.
In addition, such a polar synthetic rubber like an epichlorohydrin rubber has characteristics where a large amount of non-crosslinked low molecular polymeric substance exists at a surface of the rubber. Therefore, if a charge roller having a rubber outer surface made of such polar synthetic rubber is in contact with a photosensitive drum during stoppage of operation of image forming apparatus, the low molecular polymeric substance constituting the charge roller is likely to be migrated to a surface of the photosensitive drum. As a result, an abnormal phenomenon may occur in that an image may not be formed onto the surface of the photosensitive drum.
As a method of suppressing such an abnormal phenomenon, efforts have been made to maximize the crosslinkage density of the outer rubber surface. However, this method still increases durability of outer rubber surface while reducing the above-mentioned abnormal phenomenon, and it will be difficult to reduce the size of the charge roller.